1. Field of the Invention
The invention relates to an emergency elevator brake. In particular, this invention relates to a traction sheave brake which incorporates a friction member which is used to develop a braking torque when compressed against the traction sheave. The brake is intended to provide: (1) ascending car overspeed protection; (2) ascending car uncontrolled low speed protection; and (3) descending car uncontrolled low speed protection.
2. Background of the Invention
In conventional electrically powered elevator systems, an elevator car assembly is suspended within an elevator shaft. This assembly usually consists of an elevator car which is connected via a number of hoist ropes to a counter-balancing mass. The hoist ropes are mounted about one or more rotatable pulley-like members, where at least one such member is a traction sheave. Rotation of the traction sheave is controlled by electro-mechanical means. It is to be noted that a change in the vertical position of the elevator car caused by rotation of the traction sheave results in an equal but opposite change in the vertical position of the counter-balance. The mass of the counter-balance may vary. Commonly, the mass of the counter-balance is chosen to be approximately equal to the mass of the empty car plus the combined mass of the elevator passengers when the car is filled to half capacity.
Although conventional electrically powered elevator driving systems (machine, brake, motor, control etc.) as described are known to operate reliably some problems may be encountered. The failure of the driving system to operate in its intended manner may cause unwanted and uncontrolled rotation of the traction sheave and consequently elevator motion control may be lost.
In the absence of motion control due to the failure of the driving system to operate in its intended manner, the behaviour of the elevator system will be dictated by the operation of the gravitational forces acting on the counter-balance and the elevator car. Where the combined mass of the elevator car and the passengers is less than the mass of the counter-balance a loss in system control may cause the counter-balance to descend and the elevator car to ascend. Should this condition not be detected and rectified i.e. if there is no way to deal with this emergency, the counter-balance will continue to accelerate in the downward direction and the elevator car will continue to accelerate in the upward direction, with the possibility that the elevator car will crash into the overhead of the elevator shaft. This potentially dangerous condition is known as an "ascending car overspeed condition". The term "overspeed" refers to movement of the elevator car at a velocity greater than the rated speed of the elevator, also known as the "contract speed".
Uncontrolled low speed activity is also seen to occur. The term "low speed" is used to indicate elevator speeds on the order of the "leveling speed" which speeds are reached when the elevator is proximate to the landing area. The leveling speed is a fraction of the contract speed. It is assumed that the failure of the driving system to function in its intended manner will occur with the elevator at the door levelling zone or at floor level.
The direction of elevator car movement during uncontrolled low speed conditions will depend either on the direction of traction sheave motor operation or, upon lapsing of the driving system on the relative combined mass of the elevator car plus passengers as compared to the mass of the counter-balance. Where the mass of the counter-balance is greater than the combined mass of the elevator car plus passengers and the driving system is ineffective, the elevator car will rise slowly in an uncontrolled fashion. Where the combined mass of the elevator car plus passengers exceeds the mass of the counter-balance and the driving system fails to function in its intended manner, the elevator car will descend slowly in an uncontrolled fashion.